Two swimmers 'A'&'B', one located on one side and other on the another side of a river are situated at a distance 'D' from each other. Line joining them is making an angle theta  with the direction perpendicular to flow. The speed of each swimmer with respect to still water is 'u' & speed of river flow is v_r . Both A&B start swimming at the same time in the direction parellal to line AB towards each other and they keep on swimming in same direction. Then
(a) Find the time after which they will meet. 
(b) Find the speed of river (v_r) in terms of 'u' so that the path of the two swimmers with respect to the ground becomes perpendicular to each other.